function fib(x)
   if x < 0
      raise ParamError(description|"Negative argument invalid", extra|"Fibbonacci sequence is undefined for negative numbers")
   else
      return (function(y)
         if y == 0
            return 0
         elif y == 1
            return 1
         else
            return fself(y-1) + fself(y-2)
         end
      end)(x)
   end
end


try
>fib(2)
>fib(3)
>fib(4)
>fib(-1)
catch in e
> e
end